The information revolution has progressed rapidly with technological innovation, mainly with innovation in information processing, and the variety of uses of displays, for example, for personal computers and mobile devices has increased at workplaces and homes. Accordingly, the frequency and time of use of displays have increased dramatically.
There have been demands for higher resolution and lower power consumption of small and middle-sized displays used for mobile devices and the like.
For example, a conventional liquid crystal display device includes a transistor using amorphous silicon, polycrystalline silicon, or the like. Since the off-state current of such a transistor is about 1 pA, display can be held only for 20 ms to 30 ms. Thus, images need to be written 60 times or more per second. Such write operation is perceived as a flicker by a user and causes eyestrain.
Moreover, a liquid crystal display device using an oxide semiconductor has been developed in recent years. The off-state current of a transistor using an oxide semiconductor is extremely low and can be less than 1 zA, so that the off-state current of the transistor is almost negligible. In driving a liquid crystal display device including a transistor using an oxide semiconductor, for example, in a structure disclosed in Patent Document 1, the number of write operations (refresh operations) of signals for the same image is reduced when one image (still image) is continuously displayed, whereby power consumption is reduced.